The ullage (space above a liquid fuel) of a fuel tank typically contains a mixture of fuel vapors and air. One method of reducing the risk of combustion in a fuel tank is to keep the oxygen concentration below a specified maximum concentration. FAA regulations introduced in 2008 require an oxygen concentration of 12% or less in some fuel tanks of some passenger aircraft. For military aircraft, the maximum oxygen concentration is typically lower, for example 9%.
Inerting systems decrease the probability of combustion in a fuel tank by displacing some of the oxygen with an inert gas, typically nitrogen. One inerting system in use in aircraft is known as the On-Board Inert Gas Generation System (OBIGGS). This system uses pressure from the aircraft engine compressor to drive air through a hollow fiber membrane module. The membranes are nitrogen selective and produce oxygen enriched air (OEA) and nitrogen enriched air (NEA). The system runs continuously while the engines are running. The NEA flows into the ullage and displaces air and vapors which are vented from the ullage to the atmosphere. A pressure swing adsorption (PSA) based NEA generator has also been proposed.
U.S. Pat. No. 8,256,524 describes a fire protection system which has a fuel cell for producing a nitrogen-enriched cathode exhaust air. The exhaust air is supplied to a room to be protected. A regulating or control unit controls at least one of an air supply of a cathode of the fuel cell, a fuel supply of the anode of the fuel cell, and the supply of nitrogen-enriched cathode exhaust air into the room.
Introduction
The following paragraphs intended to introduce the reader to the detailed description to follow and not to limit or define any claimed invention.
This specification describes an alternative system for generating nitrogen enriched air (NEA). The system may be used, for example, to inert a fuel tank, particularly a fuel tank on an aircraft. In some cases, the system can operate independently from the aircraft engines. Optionally, the NEA may be used to suppress fire in other parts of an aircraft, for example a cargo hold.
The NEA generation system uses a fuel cell power module (FCPM), for example a polymer electrolyte membrane (PEM) based FCPM. The FCPM also generates electricity, which may be used to supply power to any electrical load on the aircraft. The FCPM also generates NEA which can be used to inert a fuel tank or suppress fire as described above.
In one system, a portion of the FCPM exhaust air is recirculated to its air inlet. This causes the FCPM to exhaust air with a lower oxygen concentration. However, a required flow rate through the air side of the FCPM is maintained. The rate of recirculation of air exhaust to air inlet can be controlled so as to provide exhaust air with an oxygen concentration below a selected maximum under various operating conditions. The maximum may be varied according to the need for inerting.